1. Technical Field
The present invention relates to a display device and in particular, relates to a structure of a connecting substrate which connects a driver chip mounted on a display panel and a timing controller formed on a signal substrate.
2. Background Art
A display device is used for an audio visual apparatus and an office automation apparatus. A liquid crystal display includes advantages such as thin thickness, light weight, and low power consumption. Therefore, it is widely used for a display device. The liquid crystal display includes a liquid crystal panel, an integrated circuit for drive (driver chip), an external signal substrate, a connecting substrate and the like. In the liquid crystal panel, liquid crystal is sandwiched between two substrates. In one substrate, a switching element such as a TFT (Thin Film Transistor) is formed in a matrix shape, and in another substrate, a color filter (CF), a black matrix (BM) and the like are formed. The integrated circuit for drive is mounted on a substrate having the TFT of the display panel, or mounted on a flexible substrate. The connecting substrate connects the liquid crystal panel and the external signal substrate.
Here, because a large number of lines are formed in the liquid crystal display, EMI (Electro Magnetic Interference) easily occurs. For this reason, a differential signal is used for a control signal that controls the driver chip in the liquid crystal display. LVDS (Low Voltage Differential Signaling), RSDS (Reduced Swing Differential Signaling) and the like are known as a specification for differential signal transmission. As shown in FIG. 6, a pair of signal lines is used for differential signal transmission. One of two signal lines is set as a line on a positive side and another is set as a line on a negative side. A potential difference between the two signal lines represents a signal level. In order to make each of electrical lengths of two signal lines equal, the two signal lines are connected to two terminals that are adjacent to each other and a termination resistor 5 is arranged to make a potential difference on a receiving side. Having an improved resistance to noise compared with a single end transmission system, such configuration is suitable for a high speed data transmission, and therefore is widely used for many applications.
A connecting substrate of a liquid crystal display in a related technology which uses a differential signal as mentioned above will be described with reference to FIG. 7, FIG. 8A and FIG. 8B. FIG. 7 illustrates a configuration of a part of a liquid crystal display employing a Chip on Glass (hereinafter, referred to as “COG”) configuration in which a driver chip is mounted on a substrate of a liquid crystal panel. FIG. 8A is a plan view showing a configuration of a connecting substrate corresponding to a driver chip excluding a driver chip for a termination in a connecting substrate of a related technology. FIG. 8B is a plan view showing a configuration of a connecting substrate corresponding to a driver chip for a termination in a connecting substrate of a related technology.
As shown in FIG. 7, in the liquid crystal display using a related technology, a plurality of driver chips 3 are mounted on an edge area of one substrate (for example, TFT substrate) of substrates which the liquid crystal panel 2 is composed of. A differential signal which is generated by a timing controller 8 in an external signal substrate 7 is inputted into the plurality of driver chips 3 via one connecting substrate 9. As shown in FIG. 8A, a positive line and a negative line transmitting a differential signal are directly connected to a first to a n-1 th drivers of the above-mentioned driver chips 3 respectively. As shown in FIG. 8B, with respect to an n-th driver placed in a position which is the most far from the timing controller 8, a differential signal is terminated by the termination resistor 5 which is placed between the positive line and the negative line and mounted on the connecting substrate 9 and inputted to the n-th driver. In order to reduce influence of impedance and a noise in a transmission line, the termination resistor 5 is placed near an end of the line in a receiving side. When a plurality of drivers is simultaneously driven, it is desirable that the lines are arranged in an L shape or a T shape and the termination resistor 5 is placed at a signal end.
A TCP (Tape Carrier Package) configuration in which a driver chip is mounted on a polyimide resin film having copper lines is also known as a method for mounting a driver chips. A liquid crystal display using the TCP method is disclosed in, for example, Japanese Patent Application Laid-Open No. 2004-102259. In the bulletin, a display device which includes a first and a second line, a termination resistor and a differential signal reception unit is disclosed. The first and second lines transmit data of a differential signal transmission system. The termination resistor defines a voltage according to a difference voltage of a differential signal transmitted by the first and second lines. The differential signal reception unit receives data according to a voltage defined by the termination resistor and converts the data. In the display device, the differential signal reception unit and the termination resistor are formed on the same integrated circuit.
Here, whole line impedance including the connecting substrate 9 for a differential signal differs depending on a display size or component layout of a liquid crystal display. Thus, a resistance value of the termination resistor 5 for making a matching may be selected depending on a condition of the liquid crystal display.
However, in a liquid crystal display employing the COG configuration that is a related technology shown in FIG. 7, lines connecting the first to the n-th drivers and the timing controller 8 are formed on the single connecting substrate 9, and the termination resistor 5 near the n-th driver is also formed on the connecting substrate 9. For this reason, when a resistance value of the termination resistor 5 is varied, the whole connecting substrate 9 needs to be replaced, and an unnecessary cost occurs.
Since a liquid crystal display using a TCP configuration shown in the above-mentioned bulletin (Japanese Patent Application Laid-Open No. 2004-102259) includes a configuration in which a differential signal reception unit and a termination resistor are formed on the same integrated circuit, changing a resistance value is difficult compared with the termination resistor 5 which is formed on a mounting board. Thus, it is difficult to flexibly select a resistance value. In the TCP configuration, since other two kinds of integrated circuits are required for termination, unnecessary cost for management of mounted components, change of a manufacturing process and the like inevitably occurs.
A liquid crystal display panel in which a driver chip is mounted on a glass substrate by using a COG configuration is disclosed in Japanese Patent Application Laid-Open No. 2006-066674. A differential line driver having a termination resistor is disclosed in Japanese Patent Application Laid-Open No. 2005-503073 (Published Japanese translation of WO2003/024040). A liquid crystal display separately having a first common signal line substrate and a second common signal line substrate is disclosed in Japanese Utility Model Registration No. 2539438 bulletin. A display device in which a discrete circuit component is mounted on a component mounting substrate is disclosed in Japanese Patent Application Laid-Open No. 2003-066861. A liquid crystal display in which a termination resistor is provided on a substrate is disclosed in Japanese Patent Application Laid-Open No. Hei-10-260391 (1998-260391). A matrix type display device in which the same display substrates can be used and cost reduction can be achieved by standardizing members is disclosed in Japanese Patent Registration No. 3660216.